Sharks are representatives of the oldest vertebrate class having an adaptive immune system grounded on immunoglobulins (Ig), T cell receptors (TCR) and the major histocompatibility complex (MHC), and with VDJ gene rearrangement and somatic mutation mechanisms. Attributes of the shark immune system, in comparison with that of mouse/man, reveal common features indispensable for defense against pathogens. We will study characteristics of the shark immune system that are of general interest to all immunologists, specifically the elucidation of secondary humoral immune responses in secondary lymphoid tissues (in the absence of germinal centers) and the expression of "innate" Ig receptors early in ontogeny. To accomplish this, we will study 3 molecules that we discovered in the nurse shark: 1) NAR (nurse shark or new antigen receptor), the genes of which mutate at very high levels and that is proposed to be a major player in secondary responses; 2) IgW, which is expressed in 2 major forms and is proposed to cater to specific immune challenges; and 3) IgM(1gj), which is expressed early in development and is encoded by a V gene that does not undergo rearrangement. These 3 receptors (along with conventional IgM) are proposed to provide all of the innate and adaptive functions of the shark VDJ-based humoral immune system. The Specific Aims are: 1) Reveal the basic organization of the nurse shark immune system. We will define structures of primary and secondary lymphoid tissues and examine expression and co-expression of antigen receptors in each organ; 2) Examine the function of the secreted antigen receptors, especially in secondary responses. Serum responses of each antigen receptor will be followed and affinity maturation, correlated with somatic mutation and development of lymphocytes in secondary lymphoid tissues, will be studied by phage display; 3) Study the ontogeny and function of germline joined VDJ genes, with the hypothesis that they serve an important role in defense or homeostasis early in development.